This application claims the priority of Korean Patent Application No. 2002-24994, filed May 7, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical fiber amplification method and apparatus which control a gain, and more particularly, to an optical fiber amplification method and apparatus which control a gain so that optical power of a wavelength division multiplexed (WDM) optical fiber amplifier in which channels are added/dropped is substantially constant.
2. Description of the Related Art
In a wavelength division multiplexing (WDM) method, a plurality of light signals having different wavelengths are transmitted via one optical fiber. In the WDM method, since a light signal having multi-wavelength optical channels is used in the light transmission, a wide bandwidth provided by the optical fiber can be efficiently used. Accordingly, the WDM method is widely used as a next generation light transmission technique.
An erbium-doped fiber amplifier (EDFA) is used to amplify a light signal in a WDM transmission system. An erbium-doped fiber (EDF) is made by doping an optical fiber with erbium (Er3+). The light signal is amplified by energy generated when a laser pumps erbium so that erbium ions are excited and returned to an original energy level.
In the WDM transmission system, the number of channels of light signals varies due to a system capacity change, a transmission channel error, and channel addition/dropping caused by the reconstruction of a transmission network. Due to the characteristics of the EDF used as a gain medium in the EDFA, survival channels being operated, i.e., remaining optical channels among a plurality of optical channels, are transited to a steady state through a transient state by the EDFA, and gain and light output of the EDFA are changed instantly, resulting in light transmission service errors.
A light transmission system, which transmits a light signal over a long distance, generally includes a plurality of EDFAs. Thus, although an output light fluctuation is very small in each of the EDFAs, the transmission of light signals via the plurality of EDFAs causes serious light signal errors. This is because an EDF has a gain in-homogeneity characteristic and a cross gain saturation characteristic. A variation in gain of survival channels when changing the wavelengths of the survival channels is referred to as the gain in-homogeneity characteristic. Adjusting a predetermined gain value shared by a plurality of channels in accordance with changes in the number of channels is referred to as the cross gain saturation characteristic. Due to the gain in-homogeneity characteristic and the cross gain saturation characteristic of the EDF, gain becomes varying according to the wavelengths of the survival channels and gain distribution of the survival channels. As a result, a gain control method for compensating for power inequality of each channel is required.
There are three methods of controlling the gain of the EDFA. In the first gain control method, extra channels additionally operate in a wavelength band different from a multi-channel wavelength band being operated to adjust population inversion of an EDF so that a gain can be controlled. However, in this case, as the number of channels being operated increases, the extra channels require the higher maximum power than ever and noise due to a non-linear effect can be made in a multi-channel light signal being operated.
In the second gain control method, a portion of light beyond a wavelength band of a multi-channel light signal being operated is optically fed back to induce lasing so that population inversion is maintained to optically control a gain. In this method, the power of lased light is transiently damped and oscillated due to a variation in the power of an input light signal. This phenomenon occurs when the upper-level lifetime of erbium ions contributing to lasing as a gain medium is longer than the lifetime of photons in a cavity and then balanced population inversion is transiently perturbed. If this phenomenon is not removed or is not controlled to an appropriate level or less, this phenomenon affects the survival channels.
In the third gain control method, the gain of an EDFA is controlled by detecting an input light signal to adjust the power of excited light to a proper level. Although this method can be easily accomplished in respect of cost and operation, a gain control range gets larger in proportion to the number of channels used in the light transmission, and a high speed control circuit that responds faster as the number of amplifiers increases is required in a long distance transmission system.
As a conventional invention for automatic gain control and automatic level control, “Gain-shifted EDFA With All-optical Automatic Gain Control” by M. Artiglia, ECOC'98, pp. 293–294, 1998, discloses a method of uniformly maintaining population inversion in an EDF by feeding back a portion of output light. However, this invention is unsuitable for an EDFA having a high gain because the portion of light is fed back, and the gain in-homogeneity characteristic of the EDF is not considered.
Accordingly, an EDFA considering a gain in-homogeneity characteristic and a cross gain saturation characteristic is required to obtain a wide gain bandwidth and a high power characteristic.